Bring the GL UI to Windows

This displays the OpenGL UI under Windows, inside the existing Win32 one.

This will look way better with the UI redesign I'm planning but works fine now too. Doing this is really a prerequisite for the redesign so I can work fast using Visual Studio. Also, having the GL UI available directly on all platforms lets us easier add overlays and things like quick save state slot choosers and all kinds of fun stuff.

I haven't extensively tested this yet so things will be a bit broken but you can play games, load and save states, and debugging still works (mostly).